RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
65 
assume approximately constant dimensions, and at the end of that time the upper 
surfaces of the rails were scraped to obtain flat surfaces and to ensure these surfaces 
lying in the same horizontal plane. Two special fitters from Sir W. G. Armstrong, 
Whitworth & Co. carried out this part of the work, and also superintended the 
alignment of the parts of the machine. The under surfaces of the castings 
supporting the fly-wheel, discs, &c., were scraped plane at the Elswick Works and the 
alignment of the parts was, in consequence, a comparatively easy task. Originally, 
the couplings were intended to be flexible ones, but after a few runs of the machine 
rigid couplings were found to be better and such were used. Much of the work 
connected with the couplings and other fittings to the machine was carried out in the 
Engineering Department of the Laboratory, and we are greatly indebted to 
Dr. Stanton, the Superintendent, for much advice on these matters. 
At first there was trouble with the bearings. The clearance allowed was very small, 
and after the machine had run for one or two hours the expansion of the shaft was 
sufficiently great to cause a collar to come into contact with one of the bearings. 
Increased clearance was allowed and the difficulty disappeared. From that time 
(April, 1911) no portion of the machine has given the slightest trouble. The parts 
are so exactly balanced that the tremor of the concrete base is almost too small to be 
detected. This is fortunate, as during the observations for an absolute measurement 
of a resistance, two microscopes are mounted on the concrete bed and used to gauge 
the distance apart of two coils within a thousandth of a millimetre. These measurements 
were frequently made when the speed of the machine was 1040 revolutions per 
minute, but an accuracy within a thousandth of a millimetre was obtained with little 
trouble. During the progress of our work the machine has been admired by many 
hundreds of visitors to the Laboratory, and the kindness of Sir Andrew Noble in 
having the heavy metal work carried out at Elswick is greatly appreciated. 
To place the cylinders in position on the cradles, the portions of the shaft supporting 
the discs were removed from their bearings, the cylinders threaded over the shafts 
into approximately correct positions and the shafts restored to their proper places. To 
prevent damage to the cylinders during this operation, a framework of wood was built 
to support the shafts and cylinders and to enable them to be lifted as one piece. The 
cradles on which the cylinders are supported are provided with stops and clamping 
screws to prevent any marked relative movement of a cylinder and the cradle on which 
it rests. 
Section 15.—Insulation Tests. 
When making an absolute measurement of resistance, the greatest difference of 
potential between any portion of the circuit and the earth was about 130 volts, and 
the greatest difference of potential between neighbouring turns of wire on any one 
cylinder was about 16 volts. 
The insulation resistance between the circuit and the earth was tested each day on 
which measurements of resistance were made. For the insulation test, the earth wire 
VOL. CCXIV.-A. 
K 
